What is the root cause of ferroresonance in voltage transformer? How to avoid the ferroresonance in ungrounded systems?

It is the condition when out system is at low power-factor say around0.5-0.6, that time var is more & Xl  becomes = Xc. So in Low P.f condition Voltage shoots-up, may cause fetal damage to the sysyem.

Remedial action :- Automatic Harmonics Power Factor Control panel to improve Power Factor of our system, also improves stability.

2nd :- By providing Zig-Zag transformer grounding of Power-System, it absorbs surges / Transient Over voltages

To install grounding system to avoid the ferroresonance of the transformer